Telephone system



9 Sheets-Sheet 1 March 19, 1940. c. E. LoMAx TELEPHONE SYSTEM March 19, 1 940. c. E. LoMAx TELEPHONE SYSTEM Filed .1px-'i1 4. 193e 9 Sheets-Sheet 3 b .l IIIIIL March 19, 1940.

c. E. LoMAxQ, r TELEPHONE SYSTEM Filed April 4. v1938. 9 Sheets-Sheet 4 ATTORNEYS.

March 19, 1940. c. E. LoMAx TELEPHONE SYSTEM Filed April 4, 1958 9 Sheets-Sheet 5 ATTORNEYS.

March 19, 1940. c. E. LoMAx TELEPHONE SYSTEM Filed April 4, 1938 9 Sheets-Sheet 6 mln QWN

INVENTOR.

u CLARENCE E. LOMAX BY/@/vw, M,

ATTORNEYS.

Match 19, 1940. c. E. 'I oMAx TELEPHONE SYSTEM Filed April 4. 1938 9 Sheets-Sheet 7 c. E. LoMAx 2,193,973l

TELEPHONE SYSTEM Filed April 4. 1938 9 Sheets-Sheet 8A March 19, 1940.

Much 19, 1940.. c. E. LoMAx l TELEPHONE SYSTEM Filed April 4. 1938 9 Sheets-Sheet 9 Patented Mar. 19, 1940 PATENT OFFICE aisasrs p TELEPHONE SYSTEM Clarence Ernest Lomax, Oak Park, Ill., assigner to Associated Electric Laboratories,

Inc.,

Chicago, Ill., a corporation of Delaware Application April 4, 1938, Serial No. 199,797

36 Claims.

The present invention relates to signaling systems and has for one of its objects the provision of an wherein automatic switch control impulses and auxiliary control pulses and signals are transmitted by way of carrier current channels for the purpose of controlling automatic line switching equipment.

It has heretofore been proposed to utilize carrier current channels including commercial current power transmission lines for signaling and communication purposes. The use of high voltage commercial current'frequency lines for this purpose is extremely advantageous in certain applications because it eliminates the cost of constructing and maintaining telephone lines. In the usual arrangement of this character, manually controlled switching equipment is utilized exclusively for setting up connections between the various telephone lines of the system.

It is an object of the present invention to provide an improved automatic telephone system wherein commercial frequency current power transmission lines are utilized for signaling and communication purposes, and wherein automatic switching equipment provided in a regular automatic exchange may be controlled by high frequency impulsing over a power transmission line forming a link between the automatic exchange and another exchange or a remotely located automatic telephone substation.

It is another object of the invention to provide a system of the above character wherein calls may be routed by way of the power transmission 35 line in either of two directions, one or more twoway repeaters being utilized for controllingy the carrier current equipment for operation in routing calls in the respective two directions of call transmission.

It is a further object of the invention to provide an arrangement of the character described having included therein improved apparatus wherein the functions auxiliary to line switching operations of an automatic telephone system and in-` cluding the transmission of dial, busy and ringback tones and the automatic ringing of a called substation are all provided.

Three embodiments of the invention are described in detail hereinafter, all of which are commonly characterized as comprising a system including a power transmission line having av source of commercial frequency current connected thereto. and adapted to be used as a carrier current link between the telephone lines of the v system. High frequency carrier current transimproved automatic telephone systemv (o1. irs-2.5)

mitters and receivers are provided which are located at two separated points, are coupled to the transmission line by apparatus which is operative to block the commercial frequency current therefrom and are 'so connected and arranged that two independent carrier channels are provided for transmitting, as modulation components of the high frequency carrier currents, audio frequency currents over the line in the respective two dii re'ctions of transmission. In each of the three embodiments of the invention a telephone line is coupled to the carrier transmitter and receiver located at one of the two points and the transmitter and receiver located at the other point are coupled or connected by way of a two-way repeater to have access to and to `be accessible to the automatic switching equipment of an automatic exchange. Variations in this general arrangement pertain to modiiications in the apparatus whereby calls may be routed by way of the carrier channels, the two-way repeater and the automatic switching equipment in an automatic exchange, to and from an operators position in a manual exchange or an automatic subscribers substation to a desired line terminating in the automatic exchange. In the third embodiment of the invention, provisions are made whereby the transmission line may be utilized as a link between two automatic exchanges, suitable 'rimproved apparatus being provided for adapting the automatic switching equipment provided in each of the two exchanges to utilize the carrier channels as a connecting link in setting up a connection and for the transmission of voice freduency currents when` the connection is established. In each of the three embodiments one of the carrier channels is utilized for the transmission of automatic switch seizing and release pulses from the calling telephone to the automatic switching equipment in the distant automatic exchange and also for the transmission of the automatic switch control impulses to one or more of the impulse responsive switches provided in the exchange. In one modified arrangement of the apparatus this carrier channel is also utilized for the transmission of ring cut-off control pulses to the distant automatic exchange. As distinguished from this one carrier channel the other carrier channel is utilized for the transmission of dial tone, busy tone and ring-back tone signals produced as an incident of the operation of the automatic switching equipment in the automatic exchange to a calling telephone station. This other carrier channel is also utilized for the transmission of ringing control pulses in the two arrangements wherein the transmission line is used as a link between a manual operators position or an automatic subscribers substation and an automatic exchange. One improved feature of this ringing control arrangement is that apparatus is provided whereby a calling automatic subscriber may repeatedly ring the called operator or substation, as the case may be, by repeatedly actuating the dial controlled impulsing device provided at the calling substation. In each of the three embodiments of the invention the two carrier channels are normally operative to transmit audio frequency energy in the respective two directions of transmission.

Further features of the invention pertain to the particular arrangement of the apparatus whereby the above and additional objects are attained.

The novel features believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further bjects and advantages thereof, will best be understood by reference to the specification, taken in connection with the accompanying drawings, in which Figures 1 to 3, inclusive, illustrate an automatic telephone system having incorporated therein certain features of the present invention; Figs. 4 and 5 illustrate, respectively, a high fre quency transmitter and a high frequency receiver adapted for use in all embodiments of the invention; Fig. 6 illustrates apparatus which may optionally be provided in combination with the apparatus shown in Fig. 1 only when the apparatus of Fig. i is combined with that shown in Figs. 9 and 3 to form a complete telephone system; Fig. 7 schematically illustrates an automatic telephone subsystem which is adapted to be combined with the apparatus illustrated in Figs. 2 and 3 to form a larger system in which a power transmission line is utilized as a carrier current link between two automatic exchanges; Figs. 8, 9 and 3, considered together, illustrate a modied arrangement of the system; and Fig. 10 illustrates a modication of the apparatus shown in Fig. 8.

Referring now more particularly to Figs. 1, 2 and 3 of the drawings, there is illustrated a telephone systern comprising a manual ofiice, indicated as including the apparatus to the left of the broken line shown in Fig. 1, and an automatic exchange including the apparatus shown in Fig. 3 and that portion of the apparatus shown in Fig. 2 to the right of the broken line. The two exchanges are interconnected by a power transmission line including a transmission line conductor |00 and having connected thereto a source of commercial frequency current schematically indicated at 10|. The transmission line |00 is indicated as being of the three-phase type, although only a single one of the line conductors is utilized for communication purposes. Located at two separated points along the power transmission line are two sets of carrier current equipment comprising two carrier current transmitters |40 and 240 and two carrier current receivers G45 and 245, all diagrammatically illustrated in the drawings. These transmitters and receivers, which are normally in operation, are so connected and arranged that two independent carrier channels of different carrier frequencies are provided for transmitting, as modulation components of the high frequency carrier currents, audio frequency currents by way of the transmission line conductor |00 between the manual office and the automatic exchange in the respective two directions of transmission. More specifically, the output carrier frequency of the transmitter |40 is different from the output carrier frequency of the transmitter 240, the receiver 245 is tuned to respond only to the output carrier of the transmitter |40, and the receiver |45 is tuned to respond only to the output carrier of the transmitter 240.

Located in the manual cnice is an operators telephone station indicated generally at |02, which station includes a repeating coil |03 at which a first telephone line |09 terminates. The operators telephone station |02 further comprises a calling jack |04, an answer jack |05, a signal device in the form of a drop |01` which is connected to be energized by way of the line |09, a dial controlled impulsing device |01, a dial key |08, and five control relays RHS, R|20, Rl25,

R|30 and R|35 connected and arranged in the manner illustrated. Of the enumerated relays, the relay R||0 functions to condition the remaining relay equipment for operation when a call is initiated at the station |02, the relay R| :I

is an impulsing relay, the slow-to-release relay R| is a call initiating relay, and the two slowto-release relays R| and R|35 are time delay relays which are primarily utilized during the release of a connection involving the operators station |02.

The end of the line |09 remote from the operators station |02 is normally coupled by means of a second repeating coil |31 to the input circuit of the transmitter |40 and to the output circuit of the receiver |45. More particularly, the secondary winding |39 of the repeating coil |31 is connected by way of a link line |4| and a line balancing impedance network |42 to the input circuit of the transmitter |40 and to the output circuit of the receiver |45. For the purpose of transmitting switch seizing and switch release pulses and of repeating impulses dialed at the operators station |02 by way of one of the carrier channels to the automatic switching equipment in the distant automatic exchange, there is provided an impulsing relay R|50 which is arranged for energization over a simplex circuit including the two conductors of the line |09. In order to apply ringing current to the line |09 on calls incoming to the operators telephone station |02, there is provided an alternating current ringing relay R|50 which is controllable by way of the other of the carrier channels in the manner pointed out in detail hereinafter.

'Ihe transmitter |40 includes an input transformer having a primary Winding |32' and the receiver |45 includes an output transformer having a secondary winding |21', both shown in dotted lines in Fig. 1. 'I'he primary Winding |32', the secondary winding |21', and the sec` ondary winding |39 of the repeater coil |31 are interconnected to form an antiside tone impedance network which includes, as a part thereof, the balancing network |42. The purpose of this arrangement is to prevent the audio signal voltages produced across the terminals of the secondary winding |21 from being modulated on the carrier generated by the transmitter |40, thereby to be retransmitted back to the transmitting party in the form of feed-back energy. This is accomplished by the arrangement illustrated, wherein audio output voltages produced across the terminals of the secondary 5A winding |21 cause equal and opposite currents to flow through the primary winding |32' of the transmitter input transformer. The voltage drops occasioned by these currents are substantially equal in magnitude and opposite in phase, so that the net input voltage to the transmitter for a substantial output voltage across the winding |21 is substantially zero. It is the purpose of the balancing network |42 to insure that the currents traversing the two halves of the primary winding |32 will be substantially equal in magnitude and opposite in phase at all frequencies within the signaling bandythus vproviding the maximum antisi'de tone effect. Specifically, the network |42 comprises four series-'connected resistors |51, |50, |69 and |10 which are connected in series with a 'parallel resistance-'capacitance circuit 'including a capacitor |1 I and the adjustable portion of a resistor |12. Bridged across the series parallel arrangement comprising the resistor |10, the capacitor |1|, and the adjustable resistor |12 is another fixed resistor |13. Connected between the lower line conductor of the line |4| and the junction point between the resistors |51 and |68 is a parallel padding or losser resistor |14. By properly proyportioning the impedance values of the respective enumerated circuit elements |61 to |14 with respect toeach other and relative to the impedance values of the'transformer windings |32 and |21' and the repeater winding |39, itis possible to secure an arrangement wherein thel currents flowing through the upper and lower halves of the winding and occasioned by voltage across the secondary winding |21' are substantially vequal in magnitude and substantially opposite in phase for all `frequencies within the signaling band. A certain amount of adjustment of the phase and magnitude of current ilowing through the upper half of the primary winding |32 ris possible by adjustment of the indicated slider arm along the resistor |12.

For the purpose of impressing the output carrier, either modulated or otherwise, of the f'transmitter |50 on the transmission line conductor |00, and for the further 'purpose of impressing 'modulated or unmodulated carrier voltages produced by operation of the transmitter 240 on the input circuit of the receiver |45, there is provided a transmission line coupling unit, indicated schematically at |15, a transmitter output coupling network, indicated at '|80`in `association with the transmitter |40, and a receiver vinput coupling network schematically indicated at |85 in association with the receiver -|45. The coupling unit -|15-is non-resonant in character and may be of any of several well-known `conventional arrangements. For example, this unit may constitute the well-known capacitance type of kcoupling unit wherein a counterpoise parallelng the conductor v|530 and capacitively coupledl thereto is relied upon rto provide coupling paths between the conductor |00 andthe receiver input coupling network |85 and between the transmitter output coupling network and the indicated transmission line conductor. The transmitter output coupling network, on the other hand, is preferably resonant in character,

being tuned to the output carrier frequency of Similarly, the receiver input coupling inetwork is resonant in character, being tuned tothe -output carrier vrfrequency of the'transmitter 240 erations.

and being arranged in any desired manner. Since the elements |15, |80 and |85 per se, form no part of the present invention, and several types of each are known to the art, these elements have been only `diagrammatically illustrated in the drawings. y

The automatic exchange illustrated in Figs. 2 and 3 has extending thereto a plurality of subscribers telephone lines, one of which is indicated at 300, and includes a plurality of subscriber controlled automatic switches for setting up connections between the various lines of the system. More particularly, each of the subscriber lines terminates in a line circuit, that to which the une son extends being indicated at 305, and a group of finder-'connector links are provided for performing the line switching op- One such finder-connector link is illustrated as comprisinga finder vswitch 3|0 and a switching-through connector switch 3|5. lFor the purpose of assigningA nder-connector links to successive calling ones of the lines terminating in the exchange, there is provided, in -accordance with conventional practice, an allotter, not

shown, which may be Aof any desired arrangel the other similar switches corresponding thereto are nonnumericalswitches, in the sense that they are ineiiective to perform `any line selecting funcfi `tions other than that of automatically selecting as numerical switches, in that they are yindivid- 'ually operative to perform particular line selecting functions in response to switch control impulses transmitted thereto. It will be understood that a plurality of finder-connector 'links are provided for performing the line switching 'functions, the number of links being determined by the maximum volume of traino the exchange is required to handle. Further in accordance with conventional'practice, each of the telephone lines terminating -in the exchange is multipled to the corresponding bank contacts of each of the finder-connector switches whereby a yplurality of links are available for lsetting up connections between each line and every other line of the system.

The end of each subscribers line remote `from the automatic exchange terminates in a subscribers substation of the automatic type, that associated with the line Y30|) being indicated lschematically at A. In brief, each of these substations may comprise ythe usual hand-set type of transmit-ter and receiver, a cradle or hook switch.

a ringer, and animpulsing 'device for transmitting switch control impulses by way `of the associated line to the automatic switching `equipment llocated in the line terminating exchange. y

The arrangement of "thelline circuits individvually terminating the telephone lines extending to the automatic `exchange may be of conventional arrangement. vcated finder land connector switches may be of any desired arrangement, although preferably veach of these switches is of the well-known Strowger type. The connector switch13|5 is'in'dicated asbeing of the vswitching-through type, provi- :i

sions being made therein orestablishing a clear metallic Acircuit through the switch-cn trunk calls. vwhereby .switch control impulses =may be .transmitted by 'way thereof to distant impulse responsive .e'quipment las, vfor example, .to Jan impulse Similarly, the schematically indirepeater associated with an outgoing interoflice trunk line.

For the purpose of routing calls from the automatic exchange by way of the carrier channels to the distant manual office, or vice versa, there is provided a twoway repeater, indicated generally at 320, which is coupled to carrier current equipment including the carrier transmitter 240, the carrier receiver 245, a transmission line coupling unit 215, a transmitter output coupling network 280 and a receiver input coupling network 285. The same antiside tone arrangement as is illustrated in Fig. l is utilized for coupling the carrier equipment to the repeater 320 vand a balancing network 242 is similarly provided for enhancing the effectiveness of the antiside tone arrangement at all frequencies within the signaling band. Since the carrier equipment shown in Fig. 2 is substantially a duplication of that illustrated in Fig. 1 and described above, no further description is deemed necessary to render clear an understanding thereof. Itis pointed out, however, that the corresponding elements of Fig. 2 are indicated by reference characters which correspond to the characters used in Fig. 1, with the difference that a hundreds digit of two is used in Fig. 2 whereas a hundreds digit of one is used in Fig. 1.

The repeater 320 comprises an outgoing link line 32| which terminates in the bank contacts of the connector switches in the automatic exchange, and an incoming link line 325 which terminates in the line circuit 330 individual thereto and by way of which access is gained to the non-numerical switches provided in the automatic exchange. The line 32| includes talking conductors C322 and C323 connected to opposite terminals of the secondary windings 216 and 211 of a repeating coil 218 having a primary winding 219 adapted to be connected by way of a link line 28| to the secondary winding 230 of a second repeating coil 231. Normally the secondary winding 238 of the repeating coil 231 is connected to a link line 292 having conductors C203 and C294 terminating at opposite terminals of the primary winding 33| of a third repeating coil 332 which includes a secondary winding 333 arranged for connection to the link line 325.

The repeater 320 also includes a group of control relays of which the five relays R340, R350, R350, R310, and R300, illustrated in Fig. 3, are utilized on calls incoming to the automatic exchange, and the six relays R200, R2 I0, R220, R225, R230 and R235, illustrated in Fig. 2, are utilized in extending a connection by way of the carrier channels to the distant manual oilice. There is also provided, under the control of the receiver 245, a pulsing relay R260 which corresponds to the relay Rl illustrated in Fig. 1 and which is operative to repeat received impulses by Way of the repeater 320 to the impulse responsive automatic switches in the automatic exchange.

Each of the transmitters |40 and 240 illustrated in Figs. l, 2, 7, 8, 9 and 10 may be of the form shown in Fig. 4. In general, the transmitter illustrated in Fig. 4 comprises an oscillator section 400 for developing high frequency current of the desired carrier frequency, a carrier frequency amplier section 4|0, and an audio frequency or signaling amplifier section 420. More particularly, the carrier current channel of the transmitter includes a vacuum tube 40| of the screen grid type, embodied in the oscillator section 400 and having output electrodes coupled to the upper portion of an inductance element 403 forming a part ol a tuned output circuit 402 which is resonant at the desired output carrier frequency. The output circuit 402 is coupled to the input electrodes of the tube 40| by means of the lower portion of the inductance element 403 and a condenser 404. 'Ihe input control grid of the tube 40| is connected to ground through a filter resistor 405. This grid is, during operation of the oscillator section, maintained slightly negative with respect to the associated cathode due to the direct voltage drop provided by rectied grid current owing through the resistor 405. The cathode of the tube 40| is also normally maintained at substantially direct ground potential through the provision of a bridge, not shown, but normally connected across the terminals 406 and 401', which bridge, as will be pointed out hereinafter in greater detail, comprises a set of armature springs of a control relay. With a direct current bridge across the terminals 406 and 401 such that the cathode of the tube 40| is at substantially the same direct potential as the associated control grid, the oscillator section 400 operates to deliver substantially its maximum carrier output voltage. In order to control the output of the oscillator section, or more generally, the carrier output in accordance with the control pulses delivered to the transmitter, provisions are made for driving the cathode of the tube 40| more positive with respect to the potential of the associated control grid, Which is tantamount to biasing the control grid of the tube negatively toward cutoii. More particularly, the positive bias for the cathode of the tube 40| is derived from a Voltage dividing resistor comprising the two elements 4|| and 4|2 connected across a source of direct voltage, not shown, but connected between the terminals 4|3. Specically, the arrangement is such that the voltage drop across the resistor 4I I is impressed on the cathode of the tube 40| when the bridge normally connected across the terminals 406 and 401 is opened. The magnitude of this bias voltage is suicient to reduce the voltage amplification through the tube 40| to a point Where the carrier output of the oscillator section 400 is interrupted, or reduced substantially to zero.

The output circuit 402 of the tube 40| is coupled to the input circuit of the carrier amplier section 4|0, which amplifier section comprises a pair of parallel connected vacuum tubes 4|4 and .z

4|5, each of the screen grid type. More specifically, the input circuit to the two tubes 4|4 and 4|5 includes an inductance element 4|6 coupled to the inductance element 403, and the carrier voltage developed across the inductance element z 4|6 is impressed between the input electrodes of the two tubes 4|4 and 4I5 through the two coupling condensers 4|1 and 4|8, suitable losser resistors 4I9 and 42| being provided for preventing the amplifier section from oscillating. For the purpose of biasing the cathodes of each of the two tubes 4|4 and 4|5 positive with respect to the respective associated control electrodes, there is provided a cathode biasing resistor 422 which is included in the cathode leg of the output circuit for the two tubes 4|4 and 4| 5. Normally only the bias voltage developed across the resistor 422 is impressed on the cathodes of the tubes 4|4 and 4|5. When, however, the direct current bridge across the terminals 406 and 401 is interrupted, the voltage drop across the re sistor 4H is also impressed upon the cathodes noted, thereby to increase the positive bias on these cathodes to a point approaching cutoff such that a' further blocking eiect on the carrier out- (itl put of the carrier channel is eiected.- 'I'he out.- put circuit of the amplier section 4|0 includes a circuit 423 tuned to the output carrierrfrequency and coupled to the input circuit of the transmitter output coupling network in the manner illustrated.

The audio frequency or signal amplifying channel 420 of the transmitter comprises a voltage amplier vacuum tube 425 of the triode type, a driver stage including` a vacuum tube 426 of the screen grid type, and a power amplier stage comprising two vacuum tubes 421 and 428, each of the screen grid type and connected in pushpull in the manner illustrated. More particularly, the input electrodes of the tube 425 are coupled to the adjustable terminals of a manually operable gain control potentiometer including a voltagedividing resistor 429 having its non-adjustable terminals coupled across the secondary winding 430 of an input coupling transformer 43|. AThis transformer is provided with a primary winding 432' having a center tap brought out to a terminal 433' and two end terminals connected to the terminals 434' and 435'. 'I'he manner in which this primary winding is connected and arranged in the antiside tone arrangement illustrated in Fig. 1 is described previously, it being pointed out in this regard that the terminals 400', 401', 433', 434' and 435' correspond, respectively, to the terminals |06', |01', |33', |34' and |35' shown in Fig. land to the terminals 256', 201', 233', 234 and 235', shown in Fig. 2. The output electrodes of the tube 425 are resistance coupled to the input electrodes of the tube 426 by a coupling network of conventional arrangement comprising the two resistors 436 and 431 and the audio frequency coupling ccndenser 438. The driver tube 426 is in turn coupled to the input electrodes of the push-pull con- A nected tubes 421 and 428 by means of a coupling transformer 439. Suitablecathode biasing resistors shunted by alternating current by-pass capacitors are provided for each of the four tubes 425 to 428, inclusive, in the conventional manner illustrated in the drawings.

Plate circuit modulation is employed for modulating the carrier output of the carrier amplier section 4H) in accordance with amplified audio or signal voltages developed across the output circuit of the audio or signaling channel 420. To this end there is provided in the output circuit of the push-pull connected tubes 421 andy 420 4a coupling transformer 445 having a secondary winding 44| included in the output cir- 4cuit of vthe parallel connected amplier tubes 4M and 4|5, in-series with the tuned tank circuit 423. Screen and anode voltages are supplied to each of the tubes illustrated in Fig. 4 from the.` source ofy direct voltage-connected between the terminals M3. A

Referring now to Fig. 5, there is illustrated a modulated carrier receiver which may be incorporated in` the apparatus illustrated in any one of Figs. l, 2, 7, 8, 9 and l0. In general, the

receiver comprises a modulated carrier amplilier 500, a detector and automatic gain control unit 550, and an audio frequency amplifier 520. More particularly, the carrier amplifier 500 comprises a. vacuum` tube 50| of the pentode type having input electrodes to whichV are coupled, by means of a condenser 502, an input circuit 503 which is tuned to the carrier frequency and is coupled to the receiver input coupling network in the manner indicated. The output electrodes of the tube 50| are coupled to an output circuit 504, also tuned to the carrier frequency, which is in turn coupled to an input circuit 505 resonant at the carrier frequency and forming a part of the detector and automatic .gain control unit. For the purpose of maintaining the proper bias on the control grid of the carrier amplifier tube 5cl, there is provided in the cathode leg of the output circuit an output biasing resistor 536 which is 'oy-passed for carrier frequency currents by a condenser 501. The detector and automatic gain control unit 5| D comprises, in addition to the tuned input circuit 505, a vacuum tube diode'5i l, and a load circuit comprising an adjustable portion of a biasing resistor A512 connected in series with a load resistor 5|3 and a filter resistor 5&4. The load circuit is shunted at two points by two carrier currenty lter condensers 5|5 and 5I5 connected in the manner shown. 'Ihe audio amplifier 520 comprises two vacuum tubes, 52| and 522, each of the pen'tode type, connected in tandem in the -order named. The iirst of these tubes has its input electrodes coupled oy means of a coupling condenser 5H to an adjustable portion of the load resistor 5|3, so that the audio voltage appearing across this portion of the load resistor is impressed between the control grid and cathode of the tube 52. For the purpose of preventing carrier frequency voltages from being impressed on this control grid there is provided a carrier frequency iilter condenser 523, which is connected between this control grid and the cathode of the tube 52|. The output electrodes of the tube 52| are resistance coupled by means of a rcoupling network comprising the two resistors 524 and 525 and a coupling condenser 526 to the input electrodes of the second audio amplifying tube 522, which latter tube has its output electrodes coupled by means of a coupling transformer 527 to the receiver output terminals 528and 523. The terminals 528 and 520 correspond respectively to the terminals |20 and |29, which latter terminals are illustrated in Fig. l as being connected in the antiside tone impedance network arrangement described pren viously. For the purpose of maintaining proper bias potentials on the control grids of the two audio amplifying tubes 52| and 522, cathode biasing resistors 530 andy53| are respectively provided in the cathode legs of the output circuits of these tubes, which resistors are by-passed for audio frequency currents by condensers 532 and 533, respectively.

As indicated above, automatic gain control is provided for maintaining the audio level at the output terminals 528 and 52S substantiallyy constant even though the amplitude of the carrier received on the input circuit 503 is varied within wide limits. To this end the direct voltage drop across the load resistor 5 i3, which varies directly in accordance with the intensity of the incoming carrier, is utilized as a bias voltage for controlling the two tubes 50| and 52|. Thus, this voltage is impressed through a filter resistor 5m on the control grid ofthe carrier frequency amplifer tube 50| and through a resistor 5|0 on the control electrode of the first audio amplifier tube 52|. The automatic gain control action thus realized is well-known in character.

The rectified carrier output voltage appearing across the load resistor 5|3 is also utilized to control a relay R560 which corresponds to the relay RIBU shown in Fig. 1 and the relay R260 shown in Fig. 2. The relay R560 is provided with a pair of armature springs which are connected to the two terminals 58| and 582 respectively corresponding to the terminals I8| and'I82 illustrated in Fig. 1 and the terminals 28| and 282 illustrated in Fig. 2. The winding of the relay R550 forms an output or load circuit for a Vacuum tube triode 540 having its input electrodes coupled to the load resistor 5|8 by means of a coupling condenser 54! and a resistance capacitance lter network comprising a condenser 502 and a resistor 543. A biasing voltage derived from the adjustable portion of a voltage dividing resistor 544 is impressed on the Cathode of the tube 540 for the purpose of maintaining the proper potential relations between this cathode and the control grid of the tube. This bias voltage is adjusted to a point such that the anode current traversing the winding of R580 is sucient to cause the operation of this relay when no rectified carrier voltage appears across the load resistor 5I3. When a carrier is being received, the rectied carrier voltage impressed on the control electrode of the tube 540 drives this electrode suiciently negative with respect to the associated cathode to reduce the anode current to a value such that the relay R580 restores. This may or may not be beyond cutoff, depending upon the design of the relay R560. Screen and anode voltages for the tubes indicated in Fig. 5 are derived from a source of direct voltage, not shown, but connected across the two terminals 545 and 546. More particularly, two resistors 5II'I and 548 are connected in series across the source, and the junction point therebetween is connected to the screen of the tube 50| for dropping the voltage n this electrode to a value below that of the voltage on the associated anode. Similarly, two resistors 548 and 550 are connected in series across the source, and the junction point therebetween is connected to the screen electrode of the tube 52| for the purpose of operating this electrode at a voltage lower than the voltage of the associated anode. It will be noted that a slight positive bias voltage, i. e., the drop across the adjustable portion of the resistor |2, is impressed on the cathode of the diode 5| I. This bias has the effect of completely cutting oiT the anode current through the diode 5|| in the absence of an incoming carrier and of maintaining the anode current substantially at zero until a carrier of substantial amplitude is received, thereby to render the receiver completely inoperative when the received carrier intensity is not suiicient greatly to predominate over the background noise.

Referring now more particularly to the operation of the apparatus illustrated in Figs. 1 to 3, inclusive, to establish a connection between one of the subscribers lines terminating at the automatic exchange illustrated in Fig. 3 and the operators station illustrated in Fig. 1, and assuming that a call intended for the operators station is initiated at the substation A associated with the line 300; when the calling party removes his receiver from its associated switch hook, a loop circuit is completed, in the usual manner, for causing the line circuit 305 to operate. In response to the operation of this line circuit, an idle one of the nder-connector links is assigned to the use of the calling line, and the finder portion thereof is set in operation to search for and seize the calling line. Assuming that the link illustrated, namely, the link comprising the finder switch 3|0 and the connector switch 3|5, is the link assigned to the use of the calling line,

when the finder portion thereof ls operated to the position corresponding to the line 300 it operates to switch the calling subscribers loop circuit through to the connector switch 3|5 and to condition this switch to respond to the switch control impulses dialed at the calling substation. When the first digit is dialed by the calling subscriber, the wiper carriage structure embodied in the connector switch 3|5 responds by elevating the Wipers embodied therein to a position opposite the level of bank contacts terminating a group of ten lines including the link line 32|. If a single such trunk line is terminated in the banks of the connector switch 3|5, a second predetermined digit must be dialed at the calling substation for the purpose of selecting this trunk linevfrom the selected group of lines. On the other hand, if all of the lines terminating at the selected level of 4bank contacts are lines corresponding to the line 32|, an automatic trunk hunting operation may be utilized for selecting an idle one of these lines. Assuming the latter case and assuming further that the line 32| is the rst available idle line of the selected group, when the wipers of the connector switching mechanism encounter the contacts terminating the conductors of this line, the operation of the switching mechanism is arrested and the calling subscribers loop circuit is switched through to the repeating coil 2'I8 in the conventional manner characterizing the switching-through type of connector switch. Also, ground is placed on the control conductor C324 of the line 32| for the purpose of marking this line as busy in the bank contacts of the other connector switches to which it extends. When the calling subscribers loop circuit is switched through to the repeater 2'I8, a bridge is placed across the two line conductors C322 and C323 to complete an obvious circuit for energizing the relay R235.

At RA236, the relay R235, upon operating, opens one point in the circuit traced hereinafter for energizing the relay R200. At RA23'I, the relay R235 completes an obvious circuit for energizing the sloW-to-release hold relay R230. Upon operating, the last-mentioned relay applies, at RA232, multiple ground to the control conductor C324 and, at RA23I, completes an obvious circuit for energizing the sloW-to-release relay R220. When R220 operates it completes, at RAZZI, an obvious circuit for energizing the relay R2 I 0.

The relay R2 I 0, upon operating, first completes, at RA2|6, an alternative circuit for maintaining the relay R380 energized. In this regard, it will be noted that R380 is normally energized over a circuit extending from ground at RA20I and by way of a parallel circuit including the upper and lower halves of the winding 238, RA2I2 and RA2|3, C293 and C294, and the upper and lower halves of the repeater winding 33|; RA34I, RC342, and the Winding of R380 to battery. When R2|0 operates, it interrupts, at RA2|2 and RA2l3, both sides of the above-traced simplex circuit. The relay R380 does not restore, however, since the above-mentioned alternative circuit is completed at RAZIB, this circuit extending by Way of C295, RA34 I, RC32, and the Winding of R380 to battery. At RA2 2 and RAZ I3, the relay R2|0 disconnects the repeater winding 233 from the conductors of the link line 292 and con nects this Winding to the conductors of the link line 28|, thereby further to prepare the talking loop between the calling subscribers substation and the operators station in the distant manual omce. At RAZIS, the relay R2|0 completes a multiple path for applying groundto the control conductor C324.

. At RA2 I4, the relay R2 I0 completes an obvious path for short-circuiting the winding of R225, which latter relay is normally energized by current flowing from battery through the resistor 221. Due to the time required for the iiux in the core of the relay R225 to die out, this relay is quite slow to restore. At RAZ I I, the relay R2I0 completes a circuit for energizing R200, this circuit extending from battery by way of the resistor 238, the operated armature RA226, RAZII, and the Winding of R205 to ground. As a result of its energiaation over this circuit, R200 operates, but stays operated only for the short time interval required for the restoration of R225. Obviously, when R225 restores, the above-traced operating circuit for`R200 is interrupted atRA226. During the interval when R200 is operated, the bridge across the terminals 206 and 201', which respectively correspond to the terminals 406 and 407 shown in Fig. 4, is interrupted at RA20I. From the preceding description and a further consideration of the transmitter showny in Fig. 4, it Will be apparent that the opening of this bridge results in the application of a high negative bias on the oscillator and amplier tubes included in the carrier channel of the transmitter 240. As a result, the carrier output of this transmitter is interrupted for the interval when the relay R200 is operated and, hence, a control pulse is transmitted by way of one carrier channel to the distant carrier equipment.

The response of the receiver located at the distant point of the transmission line will best be understood by again referring to the receiver illustrated in Fig. 5. As was pointed out above. when the carrier input to this receiver is interrupted, the voltage drop across the load resistor 5|3 included in the circuit of the diode 5| I .is reduced to zero, so that the negative bias impressed on the control electrode of the control tube y5150 is reduced to a value such that the anode current traversing the relay R560 is increased sufficiently to cause the operation of this relay. When the carrier input to the receiver is interrupted, therefore, the relay R560 places a bridge across the terminals 58| and 582. Similarly, when the carrier input to the receiver |45 is interrupted, a bridge is placed across the terminals |0| and |82 which correspond, respectively, to the terminals 58| and 582. Obviously, when the terminals 8| and |82 are shunted, a circuit is completed for energizing the alternating current relay RIIll. Upon operating, the relay R|60 operates its armatures RA|6| and RAME to project ringing current over the line |08 and through the winding I I4 of the repeater |03 in an obvious manner. The resulting induced voltage in the winding IIE of the repeater |03 energizes the drop 0r signal device it over an obvious circuit and causes this drop to operate to signal the operator that an incoming call awaits attention. f

Since the relay R200 isonly operated for a short time interval, it will be understood that only a short pulse of ringing current is projected over the line |69, after which the transmitter 240 is again conditioned to transmit its carrier output by Way of the transmission line `conductor |00 to' the receiver |45. The receiver |45 responds to the received carrier by causing the operating circuit for RI 60 to be interrupted.

As indicated previously, provisions are made whereby the calling subscriber may ring the oper- ,ators station any desired number of times by actuating the dial controlled impulsing device provided at the calling substation. More particularly, if the calling party at` the substation A fails -to get an answer from the operator within a reasonable time interval, he may again operate the impulsing device at the substation A repeatedly to open the loop circuit over which R235 is being held energized. Each time this loop circuit is opened, R235 restores to complete an alternative circuit for energizing R200, the last-mentioned circuit extending from battery through the resistor 238, RA236, RA2I I, and the winding of `interrupted in the manner pointed out above.

The receiver |45 responds to the interrupted carrier pulses by completing the operating circuit for RIGB, thus causing ringing current to again be projected over the line |00. The drop E00 responds to this current by vibrating its armature to give an audible signal to the operator that a call awaits attention.

The operator answers the call in the usual manner by inserting the answer plug `terminating her telephone set in the answer jack |05. Following this operation a talking loopis established betweenthe operators telephone set'and the calling subscribers substation, and the operator may converse with the calling subscriber as desired. In this regard, it is again pointed out that separate communication channels are provided in the respective two directions of voice frequency current transmission in so far as the link comprising the transmission line conductor |00 is concerned. More particularly, voice frequency currents produced by sound waves impinging upon the diaphragm of the operators transmitter are repeated through the repeating coil I 03 and over the line |09 to the repeating coil |31, and the audio voltage appearing across the secondary winding |39 of this repeating coil is impressed between the input terminals |34 and |35 coupled to the audio channel of the transmitter E40. By referring to Fig. 4, it will be seen that these currents are amplified in the audio amplifier section and are modulated on the carrier output of the carrier current channel of the transmitter. As indicated above, the carrier frequency output of the carrier current channelis different from the carrier current frequency output of the transmitter 240. The modulated carrier output of the transmitter |40 is impressed on the transmission line conductor |00 through the transmitter output coupling network |00 and the transmission line coupling unit |15. This modulated carrier voltage is impressed on the input circuit of the receiver 245 through the transmission line coupling unit 215 and the carrier frequency discriminating receiver input coupling network-235. By referring to Fig. 5 itwill be seen that the modulated carrier voltage is amplified and rectified and that the audio component `of the rectiiied voltage is further amplified in the audio amplifier section and impressed on the output terminals 228 and 229 which respectively correspond to theterminals 528 and 529 shown in Fig. 5. The level of the amplified output voltage is maintained substantially constant through the provision of the automatic gain control arrangement described previously. From the receiver output terminals 228- and 229 the amplified audio currents are passed through the antiside tone and line balancing network which is connected in circuit with the winding 239 of the repeater 231. This repeater functions to repeat the audio currents by way of the line 28|, the repeater 218, the line 32|, the conhector switch 3I5, the nder switch 3I0, and the calling subscribers line 300 to the receiver at the calling substation.

Voice frequency currents produced through operation of the transmitter at the calling substation A are conducted by way of the calling loop circuit to the repeater 218 and are repeated by this repeater over the line 28| and through the repeater 231 to the input terminals 234 and 235 of the transmitter 240. In the transmitter 240. the audio frequency currents are amplified in the audio channel and are modulated on the output carrier current of the transmitter in a manner which will be apparent from the preceding description. The modulated carrier current is impressed on the transmission line conductor |00 through the transmitter output coupling network 280 and the transmitter line coupling unit 215. The modulated carrier voltages impressed on the transmitter line conductor |00 are, in turn, impressed on the input circuit of the receiver |45 by means of the transmission line coupling unit |15 and the carrier frequency discriminating receiver input coupling network |85. In the receiver |45 the incoming modulated carrier voltage is amplified and rectied, and the rectined audio component thereof is amplified in a manner identical to that described above. The amplified audio voltage, which is held at a substantially constant level through the provision of the automatic gain control circuit, is impressed across the terminals I 28 and I 29 which correspond, respectively, to the terminals 528 and 529 illustrated in Fig. 5. Due to the antiside tone arrangement, including the line balancing network I42, the terminals |28 and |29, and the winding |39 of the repeater |31, only a portion of the audio voltage developed between the terminals |28 and |29 is impressed across the repeater winding I 39 and substantially none of this voltage is impressed between the input terminals |34. |35 of the transmitter I40. As a result of the audio frequency currents owing through the winding |33, corresponding induced voltages are developed in the coupled repeater winding |38 which cause currents to flow in the closed circuit including the repeater Winding IIlI, so that the audio frequency currents are repeated through the repeater |03 and are reproduced by the receiver of the operators telephone.

If the calling subscriber at the substation A is calling another subscriber having a line terminating at the manual o'ice in which the operators station |02 is located, the operator, after conversing with the calling party and ascertaining the number of the line desired or the name of the party desired, may extend the connection to the desired called line in the manner usually employed in this type of exchange.

The release of a connection as established between the substation A and the operators station |02` in the manner just described, is entirely under the control of the calling party. For the purpose of giving the operator a disconnect signal in those cases wherein she only functions as an intermediary for the purpose of extending a connection to one of the subscribers lines terminating in the manual oiiice, an arrangement is provided for ringing over the line |09 to reactuate the drop |06 when the calling subscriber hangs up. More particularly, when the calling subscriber at the substation A restores his receiver to its hook, the loop circuit, extending to the repeater 218 and by way of which the relay R235 is held energized, is interrupted, causing this relay to restore. Upon restoring, the relay R235 immediately completes, at RAZSE, the abovetraced alternative circuit for energizing R200. The relay R200, when energized over the abovetraced circuit, opens, at RA20, the shunt around the above-mentioned biasing path for the oscillator and amplifier tubes included in the carrier channel of the transmitter 240. As a result, the carrier output normally impressed on the transmission line |00 is interrupted. The receiver |45 responds to the interruption ol its carrier input by completing the circuit for energizing the relay RI 60 in the manner described previously. Upon operating, the relay RI again causes ringing current to be projected over the line |09 to energize the drop |06. The resulting operation of the drop |06 indicates to the operator that the connection has been released at the calling end thereof and that she may disconnect from the line |09 by withdrawing the plug terminating her telephone set from the answer jack |05. Following this operation, the apparatus illustrated in Fig. 1 is entirely released.

Returning to a consideration of the automatic switch train by way of which the connection was established, when the relay R235 restores, it not only completes the above-traced circuit for energizing R200 but, in addition, interrupts, at RA231, the operating circuit for the slow-to-release relay R230. The last-mentioned relay restores, after an interval, to interrupt, at RA23I, the operating circuit for the slow-to-release relay R220 and to open, at RA232, one of the paths for applying multiple ground to the control conductor C324. When R220 restores, after an interval, it interrupts, at RA22I, the operating circuit for R2I0, which latter relay immediately restores to open, at RA2I I, the above-traced alternative circuit for energizing R200; to recomplete, at RA2I2 and RAZI 3, the circuit for normally energizing R380; to interrupt, at RA2I6, the alternative circuit for energizing R380; to remove, at RA2|5, ground from the control conductor C324; and to interrupt, at RA2l4, a point in the path short-circuiting the winding of R225. Following the restoration of R2|0, the relay R200 restores and the relay R225 reoperates. Thus, the relay equipment included in the repeater 320 is returned to its normal condition. When R200 restores, it recompletes, at RA20I, the shunt across the terminals 206 and 201', thereby causing the transmitter 240 to again impress its carrier output on the transmission line conductor |00. The receiver |45 is thus caused to open the circuit for energizing RISO and this relay, upon restoring, disconnects the conductors of the line |09 from the ringing current source.

When ground is removed from the control conductor C324 in response to the restoration of R2 I 0 and at RAZ I5, the holding circuits for certain of the operated relays in the connector switch 315 are interrupted in a well-known manner, causing these relays to restore to initiate the release of the connector switch 3 I 5 and the finder switch 3I0. Thus, the switch train utilized in setting up the connection is entirely released. It will be noted that, when ground is removed from the control conductor C324, battery potential is ap- A plied thereto through the resistor 296, thereby to mark theline 32I as idle in the bank contacts of the connector switches to which this line extends. w

In order to route a call from or by Way of the operators station |02 to a substation associated with one of the lines terminating at ,the

automatic exchange illustrated in Figs. 2 and 3,

the operator inserts the plug terminating her telephone set in the calling jack il'and, in s0 doing, closes the springs 22 to complete an obvious circuit for energizing the relay Ri I0. operating, the relay RI Ill completes, at RAI I I, an obvious path for short-circuiting the winding of RI25 and completes, at RAI I3, a circuit lor energizing RI20, this last-mentioned circuit extending from ground at the switch spring IZtl, by way or" the spring E23, RAi i3, and the winding of RI20 to battery. At RAiIZ, the relay RIIi completes a circuit for energizing Rll, this circuit vextending from battery, by vway of the resistor I32, RAI I2, RA'Il, to the junction point between the upper and lower halves oi the repeater winding llt, where it divides, one branch extending by way of the upper hah of the winding IM, the upper conductor of the line IilS, RAESI, the upper half of the repeater winding S38, and the winding of Ritt to ground; and the other branch extending by way of the lower half of the winding I Ill, the lower conductor of the line i139, RAISE, the lower half of the winding i318. and the winding of RIl to ground. At Rlii, the relay Riil, upon operating, opens the bridge across the terminals llli and lill', which terminals respectively correspond .to the terminals HB6 and 407'` of the transmitter illustrated in Fig. 4 and described above. The opening of this bridge results in the application of a high negative bias on the oscillator and amplifier tubes in the carrier channel of the transmitter M0 in a `manner which will be apparent' from the preceding description, causing the carrier output of the transmitter' to be interrupted. The receiver 2435 responds to the interruption of the carrier output of the receiver Mt in the exact manner described above with reference to the receiver M5; that is, it operates to complete the circuit for energizing the alternating current relay RZll and to hold this circuit completed so long as the carrier output of the transmitter Idil is interrupted. The relay R260 responds by operating to interrupt, at RAESI, the above-traced circuit by way of which R330 is normally energized, causing the last-mentioned relay to restore. Upon relstoring, the relay R380 completes, at RABBZ, a

circuit for energizing R3lt0, this circuit ,extending from ground at RAi'iZ, by way of RA382, and

the winding of R340 to battery. Upon operating,

the relay R360 interrupts, at RAMi and RCM?, a further point in the circuit for normally energizing R380, and prepares, at RAii and RCSllZ, a point in the circuit traced hereinafter for energizing the pulsing relay R3ii.

A short time intervaly after the operation of Riiii the relay RIM, which is short-circuited in .response to the operation ci Riid, restores, to

interrupt, at RAiEland its associatedupper contact, the above-traced operating circuit for Ri E50. When Rl 50 restores it recompletes, at RAI 5I, the path shunting the terminals m0' and i0l, thereby to unblock the carrier channel provided by the carrier output of the transmitter Idil and to cause carrier current to be transmitted over the transmission line conductor 50d to the receiver 245. The receiver itil responds to this current Upon v by interrupting the operating circuit for Riti), causing this relay to restore to complete, at

RAilii, the above-mentioned circuit for energizing the pulsing relay R350, this circuit being siniilar to that traced previously for R38@ but ex- (.5

' plates a loop circuit extending to the line circuit` lThe relay Rtli, upon operating, completes, at RAilti, an obvio-us circuit for energizing Ril I15 at RAS'M, an obvious holding circuit for Ftdd. The relay R3'iil, upon operating, interrupts, at RAME, the above-traced operating circuit or Ri and completes, at RASll, an ob,- vious holding circuit for itself. 20

When the loop circuit extending to the line circuit is completed at the armature RA3`5I ofthe relay Rt, this line circuit operates to cause an idle one of the finder-connector links l to be assigned tothe use of the calling link line ,25 325. 1f it be assumed, for example, that the link comprising the finder switch 3 l ii and the connector switch SES is the link allotted to the use' of the calling link line 325, when the line circuit S3@ operates the nderswitch 3l@ is set in opera- 30 tion to search for and seize the line tt. When this lastrnentioned line is found by the finder switch titl, the loop circuit extending from the p repeater 382 is switched through v.to the connector portion of the link, namely, to the connector `35 switch 3Iii, and this connector switch is conditioned to respond to the impulses dialed at the operators station lili.

From the immediately preceding .description it will be apparent that when a call is initiated ,40

.at the operators telephone station E62, the relays Ri Iii, RI25, and Ridi) cooperate to cause a switch seizing control pulse to `be transmitted by Way of one of the carrier channels to thedistant carrier equipment comprising the receiver l26.5. This '45 control pulse is responded to by the four relays R349, R350, R360 and Rlt which cooperate to cause an idle linkto be `assigned to the use of the link line by completing the loop circuit extending to the line circuit 330. l All of the connector switches are provided with means for returning a dial tone signal over an established calling loop to the kcalling subscriber to indicate that the-v dialing operation may be started. So here, when the calling loop including y5 5 `the link line `325 is switched through to the connector switch 3&5, dialA tone current is returned over the loop and is repeated by the repeaters 332 and 231 to the input terminals of the transmitter 240. n'the transmitterl this audio frequency ,60 current is amplified and modulated on the carl rier output, so that it is transmitted by way of one of the carrier channels to the receiver M5. Inthe receiver H35 the audio dial tone component is separated from the carrier and amplified, and :65

is then delivered by Way of the output terminals i28 and E29 to the vrepeater itl. The amplied dial tone current is repeated by way of therepeater i3?, the line Illii and the repeater 403 to l the operators receiver, and the resulting oper- 79 when the relay R|25 restores to interrupt the above-traced operating circuit for R|50 it prepares, at RA|21 and its associated lower contact, an alternative operating circuit for this relay. At RA|26 the relay R|25 completes an obvious circuit for energizing the slow-to-release relay RISl,` which latter relay, upon operating, completes, at RA|3|, an obvious circuit for energizing the slow-to-release relay R|35. Following the operation of Rl35, the apparatus at the operators station is prepared to repeat impulses dialed at the operators station to the seized link located in the distant automatic exchange.

Before dialing the directory number of the desired subscriber, the operator actuates the dial key |08 to the off-normal position to place the pulsing relay R120 under the control of the impulsing springs embodied in the impulsing device |0'|. More particularly, when the key Iii is operated to the oi-ncrmal position, the springs |23 and |24 are opened and the springs |23 and |24 are closed, to complete an obvious alternative circuit for holding R! 20 operated. During the back stroke of each operation of the impulsing device |01, the impulsin'g springs embodied in this device are repeatedly opened and closed in the usual manner. Obviously, each time the springs are opened the operating circuit for R|20 is interrupted, causing this relay to restore. Hence, RI 20 follows the impulses of each digit dialed. During each open-circuit period of each impulse, or when RI20 is restored, the above-mentioned alternative circuit is completed for energizing RISE), this circuit extending from battery, by way of the resistor |32, RAI 2 I, RAI I3', RAIZI and its associated lower contact, the simplex circuit including the two line conductors of the line |09, and the winding of R|50 to ground. Thus, R|50 is energized and operates to interrupt, at RAI5I, the bridge across the terminals |06 and I 07 during the open-circuit pulse of each impulse. The transmitter I 40 responds in the exact manner set forth above; that is, during each period when the shunt across the terminals |06 and |01 is opened, the carrier normally impressed on the transmission line conductor |00 is interrupted. Hence, carrier current impulses are caused to be transmitted over the line |00 to the receiver 245. During the interrupted carrier period of each carrier current impulse, the receiver 245 operates, in the manner set forth above, to complete the circuit for energizing the alternating current relay R255), which relay operates to interrupt, at RAZGI, the abovedescribed circuit over which the pulsing relay R350 is being held energized. This last-mentioned relay is thus caused to follow the impulses dialed at the operators station |02 and, by interrupting at RA35| the loop circuit as extended to the operated connector switch 3| 5, causes the wipers embodied in the switching mechanism of this connector switch to be elevated to a position opposite the level of bank contacts terminating the group of ten lines including the desired called line. To summarize, it will be seen from a stepby-step analysis of the operations which intervene between the opening of the impulsing springs in the impulsing device |07 and the response of the relay R350 that this relay is caused to restore when the impulsing springs noted are opened.

It will be apparent from the preceding description that, when the impulses of the first digit are ended, the relay Ri 2i] remains operated, the relay RI50 restores, the transmitter Ili continuously impresses its carrier'current output on the transmission line conductor |00, the receiver 265 holds the operating circuit for R260 open, and the relay R350 remains operated to hold the loop circuit extending to the connector switch SI5 completed. Due to its slow-to-release characteristic, the hold relay R360 remains operated during impulsing. The number of impulses making up the rst digit will, of course, depend upon the directory number identifying the desired called substation. If, for example, it be assumed that the substation A is being called and that the directory number of the substation is 35, the iirst digit will necessarily comprise three impulses so that at the conclusion thereof the wipers of the switching mechanism embodied in the connector switch 3I5 will be left standing opposite the third level of bank contacts. When the first impulse is delivered to the connector switch SI5, the path for applying dial tone current to the calling loop including the link line S25 is interrupted by the control apparatus provided in the connector switch, in the usual manner. During the dialing of the impulses of the second digit, the apparatus illustrated in Figs. l, 2 and 3 operates in the exact manner just described. More particularly, the relay R350 is caused to restore to open the loop circuit extending to the connector switch 3|5 each time the impulsing springs of the impulsing device |01 are opened. In this case the connector switch noted responds by rotating its wipers over the contacts of the selected level and into engagement with the set of contacts corresponding to the number of impulses making up the second digit. In the case assumed, these wipers will be left standing in engagement with the sixth set of contacts in the third level, which contacts terminate the conductors of the called line 300. Suitable provisions are made in the connector switch SI5 for returning busy tone over an established calling loop in the event a selected called line is found busy; for projecting ringing current out over the called line to signal the called subscriber in case the called line is idle; and for returning ring-back tone current over the calling loop to indicate that the desired substation is being run-g. In this regard, it will be noted that busy tone and ring-back tone signal currents are returned over the line 325, repeated by the repeater 332 to the line 292, and from this line by the repeater 231 to the input terminals 234 and 235 of the transmitter 240, where they are modulated on the carrier current generated in this transmitter. From this point on, the busy and ring-back tone signal currents are transmitted over one of the carrier channels to the operators telephone station in the exact manner described above with reference to audio frequency dial tone current transmitted from the connector switch SI5 to the operators station.

When the call is answered at the called substation A, the desired conversational loop circuit is completed between this substation and the operators telephone station. Here again, two independent carrier channels are utilized for transmitting audio frequency currents in the respective two directions of voice frequency transmission in so far as the link comprising the transmission line conductor |00 and the two sets of carrier equipment is concerned. It will be noted that in this case the link line 292, the repeater S32 and the link line 325 are utilized in the connection rather than the line 28|, the repeater 218 and the line 32 as was the case described 75 above for a call outgoing from the automatic exchange to the distant manual ofiice.

The release of the connection as established to the substation A is entirely under the control of the operator in the distant manual olce.

More particularly, when the plug terminating the operators telephone set is withdrawn from the calling jack lili, the operating circuit for Ri lo is interrupted at the vsprings I22, causing this relay to restore to interrupt, at RAI II, the path short-circuiting the winding of Rl25 and to open, at RAE It, the operating circuit for RIZE). At RAS i3', the relay Rl ill'opens a point in the above-traced alternative circuit for energizing Riet). The relay RlZ reoperates, following the restoration of Rllfl, and completes, at RAIN, a third circuit for energizing Rl'), this circuit extending from battery by way of the resistor I32, RAItt, RAlfll, the simplex circuit including the line Ili, and the winding of R556 to ground. Simultaneously with the completion oi this circuit, the operating circuit for Ri is interrupted at RAI25.

The relay Rliitl, upon operating, opens, at RAI5I, the shunt across the terminals lili and ill'l, thereby to cause the carrier output of the transmitter Mtl to be interrupted. The receiver 2&5 responds to the interruption of the carrier current output of the transmitter lll@ by completing the operating circuit for R265) in the manner pointedr out above. Upon operating, R263 opens, at RAEtI, the above-described circuit over which the pulsing relay R35@ is being held energized, causing 'this relay to restore to interrupt, at RAZlti, the loop circuit extending to the operated connector switch 385 and, at RAi'sZ, the operating circuit for the slow-to-release hold relay Rttii. After an interval, the last-mentioned relay restores to interrupt, at RAltt, the holding circuit for RSM and, at RAS@ l, the operating circuit for R319. interrupts, at RASII and RCL-i653, a further point in the circuit for energizing R35ll and reprepares, at RASM and RCSflZ, the circuit, traced pre viously, for energizing R335.

' The time interval required for the successive restoration of the three relays RSM?, Riil and R340 forms a part of the time interval required for the successive restoration of the two slow-torelease relays Rlit and Rlll provided at the operators station i572. Thus, after an interval iollowing the reoperation oi RIM, the relay RIS@ restores to interrupt, at RAll, the operating circuit for RI35, which latter relay also restores, after an interval, to interrupt at RAN@ the above-traced alternative circuit for energizing Rlll. `The transmitter Mil responds to the res toration of Rl by again impressing on the transmission line conductor lil@ a carrier voltage to which the receiver 2&5 responds by interrupting the operating circuit for R259. The relay R268, upon restoring, reccrnpletes, at RAil, the previously traced circuit over which R339 is normally held energized, causing the last-mentioned relay to operate. It is pointed out here that the time interval required for the successive restoration of Riti) and Rit@ is substantially greater than the time interval required for the restoration of Ril, R36@ and in the order named and, hence, the operating circuit for RSSB is reprepared at RASM and RCtl and the operating circuit for Ril is interrupted atv RAME and ROME before the relay Ritt restores in response to the restoration of Rlil to'reprepare the common'portion or" the operating circuits vfor VR38@ The relay RSM, upon resto-ring,

and R350. When R380 operates, it interrupts, at RA38I, the holding circuit for R316, causing the last-mentioned relay to restore, to interrupt, at RA3'H, a further point in its holding circuit, and to reprepare, at RA3I2, a point in the operating oircuit for RMU, it being pointed out, however, that the last-mentioned circuit is opened at thev operated armature RA382 before the relay R310 restores. When the loop circuit extending from the repeater 332 to the operated connector switch 3 I5 is interrupted at RAEEI upon therestoration oi R353. the release of the switch train comprising the connector switch SI5 and the kfinder switch 3l@ is initiated. The manner in which these two switchesA are restored to normal is entirely conventional. During the release of the connection, busying ground is removed from the control conductor C324 in the connector switch SI5 and battery potential is applied to this conductor through the resistor 2%, thereby to substitute an idle marking for the busy marl:-

ing which prevailed during the time when the Y repeater 32B was occupied with the call.

As indicated in the introductory portion of the specication, the power transmission line and the associated carrier current equipment coupled thereto at two separated points may be utilized as a link for interconnecting two automatic' exchanges. `Such an arrangement is formed by combining the automatic exchange and associ- '7 with'the second auto-matic exchange and associatevd equipment illustrated partially in detail and partially schematically in Figs. 2 and 3. The apparatus illustrated in Fig. 7 in schematic form is an exact duplicate of that shown in Figs. 2 and 3. More particularly, the automatic exchange shown in Fig. 7 comprises a plurality of subscribers lines, one of which is indicated Aat Till), and a plurality of subscriber controlled auto-- matic switches for setting up connections between the various lines. Individually yassociated with the subscribers lines are substations of the automatic type which may be of any desired arrange ment, that 'associated with the line lull being indicated at B. The automatic switchingr equipment embodied in the exchange comprises line circuits'individual to the subscribers lines anda plurality of finder-connector links, one of which y is indicated as including a iinder switch lli) anda connector switch'li. Each of the connector switches has access `to a trunk or link line 'HQI extending to a two-way repeater Hill by Way ci which` access is gained to the carrier currentv equipment illustrated to the right in Fig. 7. The

lated equipment schematically illustrated in Figa.

two-way repeater 'l2ll also includes a link line '125 extending to a line circuit 739 by way of which the automatic switching equipment is rendered accessible to thecarrier equipment-on calls incoming to the automatic exchange of Fig. 7. The carrier current equipment is in all respects an exact duplicate of that shown in Figs. 2, i and 5. Thus, a transmitter 'lili is provided which corresponds to the transmitter itil shown in Fig. 2 and is of the form illustrated in detail in Fig. 4. A receiver 'M5 is likewise provided which corra sponde to the receiver M5 shown in Fig. 2 and is an exact duplicate oi" that illustrated in detail in Fig. 5. The transmitter output coupling network 188i, the receiver input coupling network l8r? and the transmission line coupling unit 'l'ii respectively correspond to the similar units 289, 2&5 and 215 shown in Fig. 2. In the modified system formed by combining the apparatus of Fig. l Awith .theapparatus illustrated in Figa? and 

